Various types of pressurized hydraulic systems are used for dispensing materials, such as hot melt adhesives. In the past, portions of the systems have remained pressurized after the system was turned off or deactivated, and pressure relief had to be accomplished by manually relieving the system. Pressure relief valves were incorporated into the systems, however, these typically operated only when hydraulic pressures reached critically high levels. To alleviate safety related or other types of concerns, and also to conform to safety requirements in various jurisdictions, specialized pressure relief devices have been developed over the past several years. For example, U.S. Pat. No. 5,715,864 (the '864 patent), which is assigned to the assignee of the present invention, relates to a pressure relief valve which is configured to open upon deactivation of the associated hot melt dispensing system. More specifically, when the system is deactivated, pneumatic pressure to the valve is simultaneously relieved and this opens a dump valve which returns pressurized hot melt adhesive to a storage tank. This relieves any remaining hydraulic pressure in the system.
While the pressure relief valve disclosed in the '864 patent operates well, various areas for improvement have been identified and are addressed by the present invention. For example, the pressure relief valve of the '864 patent includes a valve element sealed by a dynamic seal during its reciprocating movement. This dynamic seal operates in the high pressure side of the hydraulic system and, therefore, is subject to increased wear and reduced life. In addition, each embodiment of the pressure relief valve described in the '864 patent is attachable to a liquid distribution manifold, however, it would be desirable to have a more integrated pressure relief valve and manifold assembly which still allows the pressure relief valve to be easily removed and maintained and/or replaced as necessary. Finally, it would be desirable to provide a pressure relief valve in which the valve seat area is continuously flushed by the flow of liquid in the dispensing system.